Fraunhofer line discriminator systems are well known for sensing, measuring, and recording luminescence which is radiated from certain materials when stimulated by the sun. These systems operate on the known principle that sunlight contains identical Fraunhofer absorption lines before and after it is reflected from a material. In contrast, however, luminescence energy radiated from that material is broad band and contains no Fraunhofer absorption lines. Thus, luminescence produced by reflected sunlight can be sensed and measured by comparing the light level or intensity within a selected Fraunhofer absorption line to the light intensity in adjacent spectral regions (i.e. continuums) in which there is no Fraunhofer absorption line.
In their simpliest form, these Fraunhofer line discriminator systems employ a narrow passband filter tuned to pass a selected known Fraunhofer absorption line; a means to detect the intensity of reflected light within the selected Fraunhofer line; and means to detect the intensity of reflected light just outside the selected Fraunhofer line. The measured intensities are then applied to a well established, relationship to determine the intensity of the luminescence and reflectance of the measured material. For a more detailed description of these known Fraunhofer line discriminator systems and their operations, see U.S. Pats. Nos. 3,578,848; 3,598,994; 3,641,344; 3,769,516; and 4,433,245. While these known prior art systems provide measurements of luminescence, their design (especially that of large optical assemblies involved) has restricted their use to permanent installations, e.g. laboratories, or to large mobile units such as aircraft or satellites.
In co-pending U.S. application Ser. No. 746,050, filed June 18, 1985, and commonly assigned to the present assignee, a luminescence sensing and measuring apparatus (i.e. radiometer) is disclosed which utilizes a novel, simplified optical filter assembly which allows the apparatus to be of lightweight construction and highly portable thereby overcoming the use restrictions of the prior art systems. This lightweight optical filter assembly allows the apparatus to be hand carried into the field for measuring luminescence of target materials on site. To meet the desired lightweight requirement and still be functional over a wide range of Fraunhofer lines of possible interest, the elements of the primary optical filter assembly of the above-mentioned radiometer are mounted on a removable base element. This optical filter assembly is readily positioned into and removed from the radiometer during field operation. By having an individual optical filter assembly designed especially to sense the luminescence related to each of the specific Fraunhofer lines of interest, a particular assembly can be quickly interchanged into the radiometer to sense the luminescence about that particular Fraunhofer line.
More specifically, the lightweight optical filter assembly of the above-mentioned radiometer is comprised of three optical elements which are affixed in a defined relationship onto a base plate. The first element is a beamsplitter. The beamsplitter is positioned to lie in the line of sight with the light reflected from the material to be investigated when the optical filter assembly is in position in the radiometer. The combined visible reflected light and the light due to luminescense from the material is directed onto and is split by the beamsplitter. A large percentage of this light is reflected off the mirrored surfaces of the beamsplitter and continues through the second of the optical elements which, in turn, is a filter which is centered on the selected Fraunhofer line and which has a narrow passband (e.g. approximately 4 Angstroms). The light beam passing through the narrow band filter is focused by a lens system onto a first sensor which measures the intensity of said light beam. This measurement is representative of the intensity of light within the Fraunhofer line and is the "c" component of the known luminescence equation.
The remainder of the light passes through the beamsplitter and continues through the third optical element which is a filter centered on the selected Fraunhofer line and which has a broad passband (e.g. approximately 100 angstroms). The light beam passing through the wide band filter is focused by a lens system onto a second sensor which measures the intensity thereof. This measurement is representative of the intensity of the light in the continuums just outside the selected Fraunhofer line and is the "d" component of the luminescence equation.
In the radiometer described above, the adjustment or tuning of the narrow passband filter to its center frequency (i.e. selected Fraunhofer line frequency) and maintaining same during operation is an important factor in obtaining highly accurate readings of the light passing through the filter. This is due to the fact that the quantity being measured is relatively small in relation to the other values in the luminescense equation. The narrow passband filter is positioned at a shallow angle with respect to the light beam passing therethrough so that the light beam is slightly off normal with respect to the surface of the filter. The center frequency can be finely tuned as the temperature of the filter changes and maintained at its desired value by increasing or decreasing this shallow angle by moving adjustment rod by rotating a micrometer which extends through the front panel of the radiometer. The rod is in contact with a pivotable, spring biased mount on the optical filter assembly which carries the narrow passband filter. As the rod moves inward, the shallow angle is increased. Outward movement of the rod allows the spring biased mount to move in an opposite direction to decrease the shallow angle.
The narrow passband filter is comprised of optical quality glass which has such a low temperature expansion co-efficient that it would at first appear that normal changes in the ambient temperature during operation would have a negligible effect on the filter and hence, on the tuning of its center frequency. Nonetheless, this small temperature expansion coefficient does exist and it has been known that this coefficient causes changes in the tuning or alignment of the center frequency of the narrow passband filter in response to temperature changes, which, in turn, results in an error in the light readings being sensed through this filter. Compensation for this error, albeit small, is important since the overall accuracy of the measurements taken by the radiometer depends on accurately measuring small differences in otherwise relatively large numbers.
When this error associated with temperature was first dealt with, it was corrected by simply taking frequent temperature readings of the radiometer during use and consulting a calibration chart (previously prepared in the laboratory for the specific narrow passband filter in tune for the measured temperature). While effective, this procedure significantly slows field operations since the filter should be retuned with temperature changes as small as 0.1 degree centigrade which significantly affects the precision of the readings.
It can be seen from the above, it is highly desireable to provide a means for continuously sensing any temperature changes of the narrow passband filter and for quickly compensating therefor by fine tuning the narrow passband filter as the need arises without continuously consulting a chart or the like.